Polypropylene yarns, particularly continuous filament textile face yarns, are usually produced with conventional `down-the-stack` air quench extrusion apparatus. These are housed in a building several stories high with an extruder on an upper floor, air quench cabinets on the floor below, and inter-floor tubes extending down to a lower floor where the yarn is taken up onto packages. Cooled air is blown through the quench cabinets to solidify and cool the yarn.
One disadvantage that occurs is resonance in the formation of the filaments of the yarn. As the polypropylene melt is extruded through a capillary in a spinnerette, it swells out on the underside of the spinnerette and then the filament is drawn-down from such swelling. However, this drawing-down occurs non-uniformly and, in exaggeration, the filament forms like a string of sausage links: this is resonance. Subsequently, when the filaments are being fully drawn, this resonance tends to cause draw breaks in the filaments. The more pronounced the resonance, the greater the frequency of draw breaks.
Also, the point at which a filament completes its drawing-down, in the quench cabinet, to its undrawn denier varies. This can be seen as a rain drop effect when looking into the quench cabinet. This contributes to further non-uniformity.
The temperature at which the polypropylene melt is extruded is usually of the order of 500.degree. F., although lower temperatures have been tried. It is known that, in general, as the temperature is lowered, the swell on the underside of the spinnerette gets greater with an increase in resonance, and even the occurance of spin breaks at or near the spinnerette face.
The problem of resonance and subsequent draw breaks gets more acute with finer denier per filament yarns, for example yarns having an undrawn denier per filament less than 30, say less than 10 denier per filament in the finally drawn yarn. Also, with finer denier yarns the problem of denier variation from filament to filament, as well as along the length of the filament, becomes more noticable.